Assessment of US climate variations using the US Climate Extremes Index and the US Greenhouse Climate Response Index

Karl et al. (1996) developed two indices to quantify observed changes in climate within the contiguous United States, a US Climate Extremes Index (CEI) and a US Greenhouse Climate Response Index (GCRI). The CEI is based on a combination of climate extreme indicators, while the GCRI is a combination of indicators based on projected changes due to greenhouse climate change. These indices integrate changes in climate over several different temperature and precipitation measures and are likely to provide early detection of important changes in climate in the United States. The CEI and the GCRI should be useful for decision making because they provide concise summaries of changes in temperature and precipitation extremes over the US, relevant to many climate impact areas including energy and water use. The CEI is updated annually and used for operational climate monitoring at NCDC. Karl et al. (1996) noted an increasing trend in the CEI in recent decades and a significant positive trend in the GCRI during the 20th century. However, attribution of these observed changes to specific causes was not possible as they were not directly compared with climate model simulations.

An assessment of variations of the CEI and GCRI over the twentieth century has been undertaken, including comparison of the observed indices with those calculated from global climate model simulations. Some issues with the interpretation of variations in the CEI have been identified. A new version of the GCRI has been developed. Significant increasing trends have been found in the components of the GCRI associated with extreme maximum and minimum temperatures, due to fewer cold extremes and more hot extremes across the continental US. These variations are outside the range of internal climate variations simulated by climate models and are consistent with the models' responses to increasing greenhouses gases and sulfate aerosols. Hence, it is likely that anthropogenic climate forcing is contributing to changes in temperature extremes in the United States. While there have been recent changes in the components of the CEI and GCRI associated with precipitation extremes, these are not outside the range of internal climate variations simulated by the climate models.

Karl, T.R., R.W. Knight, D.R. Easterling, and R.G. Quayle, 1996: Indices of climate change for the United States. Bull. Am. Meteor. Soc., 77, 279-292.